1. Field of Invention
This invention relates to an improved electromagnetic flowmeter which is excited at a low frequency.
2. Description of Prior Art
Known electromagnetic flowmeters are generally operated by applying a magnetic field to a fluid in a direction normal to the flow thereof, simultaneously detecting a variation in an electric signal produced by the fluid, and measuring the rate of flow of the fluid, based on the detected signal. Recent electromagnetic flowmeters, which are widely used, are excited at a low frequency with trapezoidal or square waves, and have a more stable zero point than electromagnetic flowmeters which are excited with alternating current or direct current. With the low frequency excited electromagnetic flowmeter, a current, which is supplied to an exciting coil, is periodically switched between two or three normal values, and a voltage which is induced between electrodes when the exciting current becomes constant, is sampled and computed to remove adverse influences due to an electrochemically induced d.c. voltage and an offset voltage generated by the circuit. In this manner, there is produced a signal representative of the flow rate of the fluid.
Since any variation in the exciting current results in an error, the low frequency excited electromagnetic flowmeter generally uses a constant current exciting circuit. In the constant current circuit, the exciting current is controlled, so as to be constant, by a transistor so that the voltage obtained by detecting the exciting current will be equalized to a voltage setting. The transistor used for producing a constant current suffers a large power loss and fails to make the circuit smaller in size and higher in efficiency. That has been a serious deterent in providing an energy saving electromagnetic flowmeter.